U.S. patent number 4,689,262 [Application Number 06/775,386] was granted by the patent office on 1987-08-25 for electrically insulating substrate.
This patent grant is currently assigned to CTS Corporation. Invention is credited to Terry R. Bloom.
United States Patent |
4,689,262 |
Bloom |
August 25, 1987 |
Electrically insulating substrate
Abstract
An apparatus for fabricating composite porcelain substrates
employing alternating layers of wire cloth and porcelain green
sheet material is disclosed, wherein those alternating layers are
compressed toward one another at a somewhat elevated temperature
and then kiln fired at a further elevated temperature to form a
glazed porcelain surface for receiving further electrical
circuitry, for example, of the microelectronic type. Such a
composite porcelain substrate has good thermal properties of high
conductivity and low coefficient of expansion as well as good
electrical insulating properties and mechanical strength. In
particular its thermal expansion properties are well matched to
many surface mounted devices that may be placed on the substrate
thereby reducing the likelihood of thermal damage of device
connections. The substrate cost is very low in comparison to the
currently available alternatives.
Inventors: |
Bloom; Terry R. (Middlebury,
IN) |
Assignee: |
CTS Corporation (Elkhart,
IN)
|
Family
ID: |
27086693 |
Appl.
No.: |
06/775,386 |
Filed: |
September 12, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
612142 |
May 21, 1984 |
4572754 |
|
|
|
Current U.S.
Class: |
428/201; 174/256;
257/E23.006; 428/209; 428/210; 428/901; 442/16 |
Current CPC
Class: |
C03C
14/002 (20130101); C04B 33/26 (20130101); C04B
33/36 (20130101); H01L 21/4803 (20130101); H01L
23/142 (20130101); H05K 1/053 (20130101); Y10T
428/24926 (20150115); H01L 2924/09701 (20130101); H05K
1/092 (20130101); H05K 2201/09309 (20130101); H05K
2201/09681 (20130101); Y10S 428/901 (20130101); H01L
2924/0002 (20130101); C03C 2214/02 (20130101); Y10T
428/24851 (20150115); Y10T 442/126 (20150401); Y10T
428/24917 (20150115); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
C03C
14/00 (20060101); C04B 33/00 (20060101); C04B
33/36 (20060101); C04B 33/26 (20060101); H01L
23/14 (20060101); H01L 21/02 (20060101); H01L
23/12 (20060101); H01L 21/48 (20060101); H05K
1/05 (20060101); H05K 1/09 (20060101); B32B
018/00 (); C04B 035/74 (); C04B 037/02 (); H05K
001/03 () |
Field of
Search: |
;428/209,210,201,247,256,901 ;156/89 ;264/60,61 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cannon; James C.
Attorney, Agent or Firm: Flagg; Rodger H.
Parent Case Text
This is a divisional application of Ser. No. 612,142, now U.S. Pat.
No. 4,572,754, issued Feb. 25, 1986, filed May 21, 1984 for
ELECTRICALLY INSULATING SUBSTRATE.
SUMMARY OF THE INVENTION
The present invention relates generally to insulated structures for
supporting electronic circuitry and more particularly to substrates
of the porcelain surface variety for microelectronic applications,
such as thin film circuits or thick film circuits, frequently in
conjunction with other electronic components, for example, of the
high surface density mounted variety.
A wide variety of materials and devices are employed as substrates
in the microelectronics industry. Common among these are low
alkaline glasses, ceramics, sapphire, certain plastic films such as
mylar, semiconductor materials such as silicon wafers frequently
with an oxide surface, as well as more exotic materials, such as
titanium dioxide. Selection of a particular substrate material
almost invariably involves a tradeoff between the advantages and
disadvantages of the particular substrate material.
Among the ceramic substrate materials, beryllia and alumina are the
most commonly used and either may be employed with or without a
surface glaze. Alumina (AL.sub.2 O.sub.3) is most commonly used in
thick film microelectronics since it possesses good thermal
expansion properties, adequate mechanical strength, and good
electrically insulating properties, however, the thermal
conductivity for heat dissipation purposes of alumina is
significantly less than that of beryllia. Beryllia on the other
hand is quite expensive and its toxicity makes it very difficult to
work with.
Another ceramic material having good electrical isolation
properties is porcelain which is actually a subcategory of
materials within the ceramic field. Porcelain materials in the
green or unfired state have recently been fabricated as a tape-like
strip which may be positioned as desired and then fired to provide
an extremely good electrical insulator. Porcelain tapes of this
type are described in detail in my copending application serial
number 504,990, now U.S. Pat. No. 4,556,598, filed June 16, 1983
and assigned to the assignee of the present invention. Such
porcelain tape, per se, is somewhat brittle after firing and is not
a particularly good heat conductor and, thus, for some
microelectronic circuit designs, would provide inadequate heat
dissipation or inadequate structural strength. It would be highly
desirable to use this economical and electrically suitable
porcelain tape material as a microelectronic substrate without,
however, the two above noted defects.
Microelectronic circuitry substrate technology has branched from
simple glass and ceramic substrates in a number of directions. For
example, in U.S. Pat. Nos. 3,518,756 and 4,109,377, a multilayer
ceramic substrate includes conductive pathways between the ceramic
layers. Sintered multilayer substrates of glass-ceramic materials,
including circuit patterns, are shown in U.S. Pat. No. 4,413,061.
Another approach to multilayered substrates having a plurality of
electrically conductive layers forming the circuit pattern is U.S.
Pat. No. 4,296,272. U.S. Pat. No. 4,313,262 discloses a molydenum
substrate having very good properties as a heat sink. None of these
patented arrangements provides an inexpensive, general purpose
substrate with low thermal expansion, high thermal conductivity,
good electrical insulating properties and adequate structural
strength. When multilayer techniques are employed in the prior art,
the purpose is generally to provide electrical isolation between
the several conductive circuit paths.
The entirety of my copending application Ser. No. 504,990 entitled
Porcelian Tape for Producing Porcelainized Metal Substrates is
specifically incorporated herein by reference. Among other things,
that copending application discloses a scheme for the manufacture
of multilayer porcelain substrates. A single layer or multiple
layers of porcelain mixture with the flexible plastic carrier
removed therefrom, may be disposed on a substrate, the subsequent
second and third mixture layers being disposed one upon the other.
After the lamination step in which the layer or multiple mixture
layers are laminated to the metal substrate, a conductive paint may
be printed in a conductive pattern on the top porcelain mixture
layer and then dried. Next, a resistor paint may be screen printed
in a pattern overlapping portions of the conductive pattern, and
subsequently dried. Then another formed porcelain mixture layer is
disposed over the top porcelain mixture layer with the thick film
circuit disposed therebetween and laminated to the top layer,
followed by the entire combination being fired in a one step firing
process. Thus, multiple layers of porcelain mixtures each provided
from a flexible porcelain tape, can be stacked one upon another
over the subjacent substrate with thick film circuits disposed
between laminated adjacent layers of porcelain mixtures, and then
the entire combination fired in a single firing step wherein the
resulting fired thick film circuits are protectively disposed
between two porcelain layers intimately bonded one to another and
the substrate. This, of course, protects the printed thick film
circuits during firing from oxidation, both during the firing
process and afterwards when the overlying fired porcelain mixture
layer comprises an insulative porcelain layer. Alternatively, a
printed thick film circuit not subject to the effects of oxidation
need not have a porcelain mixture layer disposed thereover and may
comprise the top most layer of the construction. In any of these
methods, thick film conductive circuits may be screened onto the
overlying porcelain layer or layers after the firing step, and then
fired in another step.
The method for producing the flexible porcelain tape provides a
porcelain material that is easily handled, formed and applied to a
metal substrate whereby the flexible porcelain mixture cn be formed
to the exact shape of the corresponding metal substrate. Equally
important is the ability to readily manufacture multilayer
porcelainized metal substrates wherein the thick film electrical
circuits may be disposed between protective insulative porcelain
layers or upon the top of a plurality of insulative porcelain
layers to provide superior insulative separation and high
dielectric strength between the circuits and subjacent metal
substrate.
My copending application also discloses techniques such as the
Teflon coating of pressure surfaces applied to such substrate
materials. The metalic substrates employed in my prior application
in the specifically disclosed examples were either steel or
aluminum, however, the techniques of that application can be
extended to other materials.
Among the several objects of the present invention, may be noted
the provision of a microelectronic circuit substrate which
capitalizes on the technology set forth in the above mentioned
copending application; the provision of a substrate having thermal
expansion and structural strength properties quite similar to
alumina; the provision of a substrate having a thermal conductivity
as good as ceramic coated, copper clad invar; the provision of a
substrate of extremely low cost; the provision of a substrate
having thermal expansion properties nearly the same as those of
components to be received on the substrate; the provision of a
method of fabricating an electrically insulative substrate for
subsequent use in a microelectronic circuit; and the provision of a
general purpose microelectronic circuit substrate incorporating
sheets of wire cloth providing enhanced mechanical strength,
improved thermal conductivity for heat dissipation purposes, radio
frequency interference shielding comparable to conventional metalic
chasis, and not a part of the microelectronic circuit, per se.
These as well as other objects and advantageous features of the
present invention will be in part apparent and in part pointed out
hereinafter.
In general, a method of fabricating an elecrically insulative
substrate includes providing interleaved porcelain green sheets and
wire cloth sheets which are compressed toward one another and then
kiln fired to form a glazed porcelain surface for receiving
electrical circuitry.
Also in general and in one form of the invention, a composite
substrate has a glazed porcelain circuitry receiving surface and
comprises alternating layers of porcelain sheets and fine mesh
metal screen compressed together with the porcelain sheets
outermost.
Claims
I claim:
1. A precursor for an electrically insulating substrate having a
glazed porcelain surface for receiving electrical circuitry
thereon; comparatively low thermal coefficient of expansion;
comparatively high thermal conductivity for heat dissipation; good
electrical insulation properties; and sufficient mechanical
strength for subsequent use in a microelectronic circuit having
surface mounted devices disposed thereon; which comprises:
a first plurality of porcelain green sheets having a relatively low
linear thermal coefficient of expansion matched to the linear
thermal coefficient of expansion of the surface mounted devices to
be subsequently mounted upon the substrate; and a second plurality
of fine mesh wire cloth sheets for mechanical strength and high
thermal conductivity;
said second plurality of fine mesh wire cloth sheets interleaved
between the first plurality of porcelain green sheets to separate
the fine mesh wire sheets by at least one porcelain green sheet to
provide electrical insulation between the wire cloth sheets,
wherein the interleaved porcelain green sheets and the fine mesh
wire cloth sheets have been compressed against one another at a
somewhat elevated temperature with sufficient pressure to form a
laminated substrate, for subsequent firing at a further elevated
temperature sufficient to form at least one outermost glazed
porcelain surface.
2. The substrate of claim 1 wherein each metal screen layer
comprises copper screen of about 100 mesh.
3. The composite of claim 1, subsequently fired, further including
adhering at least one thick film conductor adhered to at least one
of the outermost glazed porcelain surfaces.
4. The composite of claim 1, further subsequently fired, including
screened and fired thick film paint adhered to at least one of the
outermost glazed porcelain surfaces.
5. The composite of claim 1, wherein the first plurality of
porcelain green sheets is ten and the second plurality of fine mesh
wire cloth sheets is five, with three adjacent porcelain green
sheets forming each of the outermost layers with alternating wire
cloth sheets and porcelain green sheets interleaved
therebetween.
6. The composite of claim 1, wherein the first plurality of
porcelain green sheets is at least one greater than the second
plurality of fine mesh wire cloth sheets and both outermost layers
of the substrate each comprise a porcelain green sheet prior to
firing.
7. The composite of claim 1, wherein every pair of fine mesh wire
cloth sheets is separated by exactly one porcelain green sheet.
8. The substrate of claim 1, wherein the fine wire mesh sheets
provide improved radio frequency interference shielding.
9. The composite of claim 1, wherein a porcelain green sheet is
screened with a desired circuitry configuration, dried and
interleaved with at least one other porcelain green sheet prior to
firing, to protectively dispose the desired circuitry configuration
between two adjacent porcelain sheets, to protect the desired
circuitry configuration from oxidation.
10. The composite substrate of claim 9, wherein the desired
circuitry configuration comprises a conductive paint.
11. The composite of claim 9, wherein the desired circuitry
configuration comprises a resistive paint.
12. A fired composite insulating substrate for mounting electrical
components thereon, which comprises:
a plurality of porcelain sheets and fine wire mesh sheets,
interleaved to insulate the fine wire mesh sheets between the
porcelain sheets, said interleaved sheets having been pressure
laminated prior to firing, wherein thermal energy generated by
circuitry and electrical components mounted on the substrate is
dissipated through at least one insulative porcelain sheet to the
fine wire mesh sheets interleaved within the substrate, said fine
wire mesh sheets providing the substrate with mechanical strength
and thermal expansion characteristics comparable to alumina.
13. The substrate of claim 12 wherein each metal screen layer
comprises copper screen of about 100 mesh.
14. The substrate of claim 12 wherein there is one porcelain sheet
between each adjacent pair of metal screen layers and additionally
three adjacent outermost porcelain sheets forming each of the
opposed substrate surfaces.
15. The substrate of claim 12, wherein the desired circuitry
configuration comprises a conductive paint and a resistive paint,
the conductive and resistive paints each separately screened and
dried to form a desired overlapping circuitry configuration.
16. The substrate of claim 12, wherein the fine wire mesh sheets
provide radio frequency interference shielding.
17. An electrically insulating substrate having a glazed porcelain
surface; comparatively low thermal coefficient of expansion;
comparatively high thermal conductivity; good electrical insulation
properties; and high mechanical strength formed by firing a
composite comprising:
a first plurality of porcelain green sheets having a relatively low
linear thermal coefficient of expansion; and a second plurality of
fine mesh wire cloth sheets interleaved between the first plurality
of porcelain green sheets to separate the fine mesh wire sheets by
at least one porcelain green sheet to provide electrical insulation
between the wire cloth sheets, wherein the interleaved porcelain
green sheets and the fine mesh wire cloth sheets are compressed
against one another at a somewhat elevated temperature with
sufficient pressure to form a laminated substrate.
18. The substrate of claim 17, wherein the first plurality of
porcelain green sheets was at least one greater than the second
plurality of fine mesh wire cloth sheets and both outermost layers
of the substrate comprised a porcelain green sheet prior to
firing.
19. The substrate of claim 17, wherein a desired circuitry
configuration was screened and dried upon at least one porcelain
green sheet prior to firing.
20. The substrate of claim 17, wherein a desired circuitry
configuration has been screened and dried upon at least one of the
outer glazed porcelain surfaces after firing.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram depiction of the method of fabricating
substrates in accordance with the present invention;
FIG. 2 is an exploded perspective view of a portion of an
illustrative substrate;
FIG. 3 is a perspective view illustrating the substrate of FIG. 2
with microelectronic conductors and circuit components received
thereon; and
FIG. 4 is a partial view in cross section along lines 44 of FIG.
3.
Corresponding reference characters indicate corresponding parts
throughout the several views of the drawing.
The exemplifications set out herein illustrate a preferred
embodiment of the invention in one form, and such exemplifications
are not to be construed as limiting the scope of the disclosure or
the scope of the invention in any manner.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to the process depiction of FIG. 1 and the exploded
perspective view of FIG. 2, sheets of wire cloth, such as an about
100 mesh copper screen having a thickness of about 10 to 12 mils
and porcelain green sheets, for example, of the porcelain tape
variety described in the above mentioned copending application, are
provided from supply sources 11 and 13 respectively with the wire
cloth sheets illustrated at 15 and 17 in FIG. 2 while the porcelain
green sheets are illustrated at 19 and 21. The sheets are
interleaved or superposed in layers at process stage 23. In one
preferred form, every pair of wire cloth sheets is separated by at
least one porcelain green sheet and most typically, by exactly one
porcelain green sheet, while the upper and lower outermost
porcelain green sheet layers are formed as three adjacent pieces of
porcelain tape. This thicker outer layer aids in reducing any
wrinkles or corrugations which might be induced due to the
irregularities in the woven copper screen. Five screen layers with
four intermediate separating porcelain tape layers and an
additional three layers of tape at the outer surfaces has been
found suitable.
Once the layers are appropriately interleaved, those interleaved
sheets are subjected to pressure at process stage 25 so as to
compress the interleaved sheets against one another, thereby
forming a laminated composite substrate form still in a green
state. This pressure may be accompanied by an elevation in
temperature, for example, to around 200.degree. F. (about
90.degree. C.) with the pressure lamination occuring at around 4000
to 5000 pounds per square inch and for a time interval of, say, 3
to 5 minutes. Under such pressure the green sheets or porcelain
tape tends to form about individual strands of the screen somewhat
as illustrated in FIG. 4.
Subsequent to the pressure lamination the composite substrate,
still in its green state, is kiln fired at 27, which again for
exemplary purposes may be performed for about a six minute interval
with the substrate temperature reaching a peak of around
700.degree. C. Subsequent to such firing, the surface which is now
a glazed porcelain surface, may be too irregular for some
microelectronic circuit purposes, in which case an optional
pressure flattening of the kiln fired sheets as illustrated at 29,
may be performed. Such pressure flattening could be by compression
between a pair of flat plates and at an elevated temperature of,
say around 670.degree. C. The thus formed substrate, either with or
without the optional pressure flattening 29, may then undergo
subsequent processing 31 of conventional type, such as the
screening of thick film paints onto the glazed porcelain tape
surface 33.
Referring now to FIGS. 2 and 4 the substrate includes three sheets
of porcelain tape 35, 37 and 39 followed by a wire cloth sheet 15
and then alternating porcelain tape and wire cloth sheets, such as
19, 17 and 21, continuing through a sequence of five wire cloth
sheets and terminating at the bottom of the substrate with another
three consecutive layers of porcelain tape.
The step of subsequently processing as depicted at 31 in FIG. 1,
may include the painting or screening of thick film conductors,
such as 41 and 43, to adhere to the surface of the substrate by
conventional techniques and, again by conventional techniques, a
plurality of surface mounted devices, such as 45 and 47, may be
affixed to the porcelain substrate as by soldering to the thick
film conductors. The good thermal conductivity of the copper allows
these components, such as 45 and 47, to be mounted on the porcelain
substrate in a high density manner while still adequately cooling
the components. Components 45 and 47 as well as the thick film
conductors 41 and 43 as illustrated in FIGS. 3 and 4 are, of
course, only for illustrative purposes since the substrate 49 is,
in a sense, a general purpose substrate and may be used for either
thick or thin film purposes or other microelectronic application as
desired.
The composite substrate, as thus far described, is particularly
useful when its linear coefficient of thermal expansion is matched
to the coefficient of surface mounted devices, such as resistors or
capacitors, to be mounted on the substrate, since under the
circumstances even though the substrate may expand due to heating,
the surface mounted devices also expand reducing the likelihood of
damage to components or connections due to that thermal expansion.
Many of the surface mounted devices contemplated for use with the
present substrate are alumina, which has a linear coefficient of
about 60 inches per inch per degree centigrade and the substrates
manufactured according to the techniques of the present invention
generally lie withing the range of 50 to 120 inches per inch per
degree centigrade with that coefficient being determined mostly by
the glass employed in the tape composition.
In the aforementioned copending application, two specific examples
of tape composition by weight, one for steel substrates and another
for aluminum substrates, were disclosed. A presently preferred tape
composition for use in the present invention is as follows:
500 g: Glass
116.7 g: Acryloid B7
45.0 g: MEK
100.0 g: Toluene
62.5 g: Acetone
12.0 g: Sanicizer 261
2.2 g: DC#3
The glass material in this typical tape composition is ELPOR 2010
available from Ferro Corporation, Cleveland, Ohio, while the
remaining constituents are quite similar to those set forth in
table 2 and table 4 of my aforementioned copending application. A
composite substrate employing these components has a coefficient of
expansion of about 82.times.10.sup.-7 /.degree.C.
Another glass which may be employed has 40% lead oxide, 20% barium
oxide and 40% silicon dioxide and when compounded, as above,
provides a composite substrate having a coefficient of expansion of
about 46.times.10.sup.-7 /.degree.C.
From the foregoing it is now apparent that a novel composite
substrate, as well as a novel method for fabricating a substrate
from alternate layers of porcelain tape and wire screen, have been
disclosed meeting the objects and advantageous features set out
hereinbefore as well as others and that modifications as to the
precise configuration, shapes and details may be made by those
having ordinary skill in the art without departing from the spirit
of the invention or the scope thereof as set out by the claims
which follow.
* * * * *